Distant Active Galactic Nuclei are of particular interest since their gamma ray spectra
are very sensitive to absorption by interaction with extragalactic background light
(see e.g. see discussion of the blazars
1ES 1101-232 or
1ES 0229+200).
They can hence be used not only to determine the level of light filling the space between
galaxies, but ultimately also to probe its variation with redshift, and the evolution
of the star formation rate in the Universe.

The object KUV 00311-1938, located at a tentative redshift of 0.61, is also known as the X-ray source 1RXS J003334.6-192130, and is
part of the sedentary survey
of highly X-ray dominated BL Lacertae objects, which includes 150 sources.
KUV 00311-1938 is a bright GeV gamma ray source, contained in the 1 FGL
and 2 FLG Fermi catalogs.
A lower limit on the redshift is determined as 0.51 (Pita et al. 2012).

H.E.S.S. observed KUV 00311-1938 between 2009 and 2011 in a campaign leading to 52.5 hours of good quality data, resulting in a statistically significant detection (Fig. 1) when
analyzed with advanced algorithms.
The tentative redshift of 0.61 - if correct - makes this the currently most distant very high energy gamma ray source.

The energy spectrum of KUV 00311-1938 indeed shows the anticipated break between the
GeV (Fermi) and TeV (H.E.S.S.) regimes (Fig. 2), likely caused by the onset of absorption, increasing the
spectral index of 1.8 in the Fermi band by more than 2 units. Within errors,
the preliminary H.E.S.S spectral index of about 4 is consistent with the expected sleep slope.
The wide-band energy distribution (Fig. 3) exhibits the familiar double-humped
structure, with a second peak between the UV and X-ray regimes, reflecting synchrotron emission of the parent electron population.

H.E.S.S. II
will allow exploring high-redshift objects and the transition regime between the Fermi energy
range and the current H.E.S.S. energy range in much more detail.

Fig. 1:
H.E.S.S. gamma ray image of KUV 00311-1938. The excess
of 182 gamma ray events at
the center of the field of view corresponds to a significance
slightly above 5 sigma.

Fig. 2:
The Fermi (GeV) and H.E.S.S. (TeV) spectra of KUV 00311-1938.
The curves labeled z=0.51 and z=0.95 indicate how absorption
by the extragalactic background light would attenuate the
Fermi spectrum at higher energies.

Fig. 3:
The spectrum emitted by the the BL Lac object across the full range from
radio to very high energy gamma rays.